1. Field of the Invention
100031 The present invention relates to one time programmable memory arrays, and more particularly to one time programmable (OTP) memory arrays that are hacker proof using current hacking techniques.
2. Related Art
FIG. 1 illustrates a conventional OTP memory cell array that is initialized to a known state at power up or reset. For example, in the OTP memory cell array 101 of FIG. 1, the known state is set to all zeros. FIG. 2 illustrates the conventional OTP memory cell array 101 with the bits flipped to the programmed state. The bit flipping causes a detectable edge transition, which can be detected by sophisticated equipment such as E-beam probes, used by hackers today.
FIG. 3 illustrates a conventional set top TV decoder box 301. The decoder box 301 includes a chip 302 that has the OTP memory cell array 101 and a processor 305 that compares encryption keys. The encryption keys are used to prevent hacking of the information stored in the system. The OTP memory cell array 101 holds one-half of the key, and an external SRAM 306 holds the other half of the key. Both keys are compared by the processor 305.
Thus, a particular problem that exists in the set top box industry is the ever increasing sophistication of hackers. Hackers often attempted to obtain unauthorized access key by accessing the key embedded in the set-top box. This typically involved the opening of the chip package, and examination of the chip under a microscope. In the early OTP memory arrays, this would easily yield the key, particularly since the key was identical for thousands of boxes.
Later, as the OTP memories became more sophisticated, so did the techniques used by hackers to discover the key embedded in the memory. Such techniques include, for example, the use of X-rays, E-beam probes, etc. Such techniques can frequently detect the rapid transition of a bit from one state to another during power up.
Conventional OTP memories can be hacked, and their contents can be discovered by several methods. A poly fuse OTP can be X-rayed and the fuses can visually be seen. A gate oxide fuse type OTP memory can be connected to an E-beam probe station and the bits can be observed at power up to determine their state—assuming the OTP memory has an initial state.
Accordingly, what is needed is a method and system that prevents a hacker from using the power up sequence to discover the secure key embedded in the OTP memory.